1. Technical Field
The present invention relates generally to an amplifier such as that with the radio frequency (RF) spectrum having a nonlinear feedback loop to cancel out distortions in the input signal, and method therefor.
2. Related Art
In amplifier design, a balance exists between distortion performance and efficiency. Linear amplifiers which operate under certain conditions create little distortion but are inefficient. Conversely, nonlinear amplifiers are reasonably efficient but are known to introduce significant distortion. While both efficiency and distortion are important considerations in amplifier design, efficiency becomes increasingly important at high power levels. Because of their efficiency, nonlinear amplifiers are largely preferred, leaving the problem of distortion to be solved.
Radio frequency (RF) amplifiers known in the art can be equipped with linearization circuitry. The linearization circuitry is typically placed in front of (i.e., predistortion), around (i.e., feedback), or after (i.e., feedforward) the amplifier. However, in each of these techniques, undesirable losses and, in some cases, increased power dissipation are incurred.
There is thus a need for a linearization circuit which can enhance amplifier linearity for both broadband and narrowband signals, while eliminating undesirable losses and reducing unwanted power dissipation.
It is therefore a feature of the present invention to overcome the above shortcomings related to increased losses and power dissipation of typical linearization circuits by providing a novel linearization circuit that modifies the amplifier without increasing either losses or power dissipation, and which has the additional benefit of not reducing the gain of the amplifier. The disclosed linearization circuit can nevertheless be applied to broadband technologies, and as such, is applicable to the entire cable television (CATV) frequency range. The disclosed linearization circuit is also expandable in frequency range so as to be useful in narrowband applications.
The present invention eliminates losses and power dissipation problems due to the linearization circuit by utilizing the knowledge that in most operating regimes the distortion in a RF amplifier is significantly smaller than the overall output power. For this reason, a feedback loop with nonlinear characteristics can be designed to cancel out these distortions without significantly affecting the response or the stability of the RF amplifier.
In high bandwidth linear amplifiers, feedback is typically provided in a single transistor stages composed of a few transistors only. Thus, the phase is well controlled and a nonlinear feedback scheme is devisable with a controlled phase as is required for broadband cancellation. This scheme reduces the amount of feedback at large signal excursions, which increases the gain at those signals and which offsets the saturation behavior of the RF amplifier that would otherwise reduce the gain. Both the small and large signals"" gains are kept equal to the small signal gain. Thus, the RF amplifier linearity is enhanced in a simple scheme without adding losses from power dissipation.
The present invention is also drawn to a method of utilizing the nonlinear feedback linearizer disclosed herein by combining the nonlinear feedback linearizer circuit of the present invention with an existing RF amplifier.
In a first general aspect, the present invention provides an apparatus for enhancing amplifier linearity over a frequency range, said apparatus comprising: an input frequency signal, said input frequency signal subject to signal excursions from a nominal signal level; circuitry which includes a circuit for generating a corrective signal having nonlinear characteristics; and a system for receiving the input frequency signal, said system including a circuit for applying a corrective signal to the input frequency signal.
In a second general aspect, the present invention provides a method for enhancing amplifier linearity over a frequency range, said method comprising: providing an input frequency signal, said input frequency signal subject to signal excursions from a nominal signal level; receiving the input frequency signal; generating a corrective signal having nonlinear characteristics; and applying the corrective signal to the input frequency signal.
In a third general aspect, the present invention provides an apparatus for reducing distortion in a transmitted signal, said apparatus comprising: at least one radio frequency amplifier; and at least one nonlinear feedback linearizer circuit operationally connected to said amplifier.
The foregoing and other features and advantages of the invention will be apparent from the following more particular description of embodiments of the invention.